1. Field of the Invention
This invention relates to a scanning optical device. More particularly, it relates to a scanning optical device realized by using an optical system comprising a diffraction optical element for focussing one or more than one light beams that is deflected by a deflection element on a surface to be scanned. A scanning optical system according to the invention can suitably be used for an image forming apparatus utilizing an electrophotographic process such as a laser beam printer or a digital copying machine that is adapted to record image information by optically scanning a surface by means of one or more than one light beams.
2. Related Background Art
Optical scanners to be used for image-forming apparatus including laser beam printers (LBPs) and digital copying machines are adapted to cyclically deflect a light beam that is optically modulated according to an image signal and emitted from a light source by means of an optical deflector such as a rotary polygon mirror, converge the deflected light beam to a spot of light on the surface to be scanned of a photosensitive drum by means of an imaging optical system having an fxcex8 feature and cause the light beam to scan the surface in order to record image information thereon.
FIG. 1 of the accompanying drawings is a schematic illustration of a known scanning optical system of the type under consideration, showing only principal portions thereof. Referring to FIG. 1, a divergent light beam emitted from a light source 91 is substantially collimated by a collimator lens 92, limited for its width by an aperture 93 and then made to enter a cylindrical lens 94 having a predetermined refractive power only in the sub-scanning direction. The substantially collimated light beam entering the cylindrical lens 94 leaves the latter, keeping the substantially collimated state in the main-scanning plane. It is, however, converged in the sub-scanning plane and focussed on a deflecting plane (reflecting plane) 95a of an optical deflector 95, which is a rotary polygon mirror, to produce a substantially linear image extending in the main-scanning direction.
Then, the light beam deflected/reflected by the deflecting plane 95a of the optical deflector 95 is led to the surface (to be scanned) of a photosensitive drum 98 by way of a scanning optical system (fxcex8 lenses) having an fxcex8 feature to optically scan the surface of the photosensitive drum 98 in the direction of arrow B (main-scanning direction) in FIG. 1 as the optical deflector 95 is driven to rotate in the sense of arrow A in FIG. 1.
A number of scanning optical devices of the above described type have been proposed and many of them use plastic resin for the lenses of scanning optical system of the device because it is possible to accurately correct the aberration of a plastic resin lens and such a lens can be manufactured at low cost by injection molding.
However, a plastic lens shows large fluctuations in the aberration thereof (particularly in terms of off-focus and variance of magnification) when the environment changes and this problem is serious particularly when the scanning optical device is made to produce a spot of light having a very small diameter.
Recently, scanning optical devices using a diffraction optical element for the scanning optical system have been proposed to compensate the fluctuations of aberration that are specific to plastic lenses. Japanese Patent Application Laid-Open No. 10-68903 describes such an arrangement. According to the patent document, a diffraction optical element is used to generate chromatic aberration in order to compensate the change in the aberration due to a lowered refractive index of a plastic lens with the change in the aberration due to the fluctuations of the wavelength of a semiconductor laser operating as light source. Additionally, a diffraction optical element provide an advantage of showing a highly uniform thickness when formed by injection molding if it is used by itself.
While a diffraction optical element is very effective when used for the optical system of a scanning optical device, it is accompanied by a problem that the efficiency of use (as defined by the quantity of light output/quantity of light input for the designed order of diffraction=xcex7, which is referred to as xe2x80x9cdiffraction efficiency xcex7xe2x80x9d hereinafter) varies depending on various conditions unlike a refraction optical element. This will be discussed below by using a diffraction grating model.
FIG. 2 is a schematic illustration of a diffraction grating model that can be used for a diffraction optical element. The diffraction optical element of FIG. 2 comprises a continuous grating showing a pitch p (xcexcm) and a depth h (xcexcm). The ratio of the pitch p to the depth h of the grating is referred to as aspect ratio AR. In other words, AR=grating pitch p/grating depth h.
The light beam striking the diffraction grating model with an angle of incidence of xcex8i is diffracted in the direction of the designed order of diffraction. However, when the grating pitch p is particularly small, the diffraction efficiency is theoretically aggravated to reduce the quantity of light for the designed order of diffraction on the surface to be scanned to make diffracted light of orders other than the designed order of diffraction (hereinafter referred to as xe2x80x9cdiffracted light of adjunctive orders of diffractionxe2x80x9d) noticeable and consequently give rise to undesired phenomena including those of flare and ghost.
FIG. 3 is a graph showing the aspect ratio dependency of the diffraction efficiency of the diffraction grating model of FIG. 2 when the angle of incidence xcex8i of light striking the grating (diffraction grating) is equal to zero, or xcex8i=0. In FIG. 3, the aspect ratio AR is made to vary by changing the grating pitch p while holding the grating depth h to a constant value. From FIG. 3 it will be seen that the diffraction efficiency falls dramatically when the aspect ratio is made smaller than 4.
FIG. 4 is a graph showing the diffraction efficiency for the operational order of diffraction and those for the adjunctive orders of diffraction of the diffraction grating model of FIG. 2 when the aspect ratio=3.4 (pitch=10.2 xcexcm and depth=3.0 xcexcm) and the angle of incidence of light xcex8i relative to the grating=23xc2x0. Note that the diffraction efficiency is computed by using a technique of close-coupled wave analysis. The operational order of diffraction refers to the designed order of diffraction. Thus, a diffracted beam of light of the order is used and focussed to form a spot of light on the surface to be scanned.
Conventionally, the profile of the grating is determined only from the viewpoint of improving the diffraction efficiency of the diffraction grating for the operational order of diffraction. This will be discussed below by referring to FIG. 5.
FIG. 5 is a graph illustrating the change in the ratio of the quantity of diffracted light of the adjunctive orders of diffraction used for exposure (relative to the quantity of diffracted light of the operational order of diffraction used for exposure) that varies as a function of the blaze angle of diffraction grating under the above condition. It will be seen from FIG. 5 that the quantity of diffracted light of the adjunctive orders of diffraction of the negative side used for exposure increases when the blaze angle is smaller than the one that maximizes the diffraction efficiency of diffracted light of the operational order of diffraction. On the other hand, the quantity of diffracted light of the adjunctive orders of diffraction of the positive side used for exposure increases when the blaze angle is greater than the one that maximizes the diffraction efficiency of diffracted light of the operational order of diffraction. Then, the quantity of diffracted light of the operational order of diffraction is maximized at or near the blaze angle that equalizes the above two quantities. Conventionally, the quantity of diffracted light of the orders of diffraction of the positive side is made equal to that of the negative side in order to maximize the diffraction efficiency of diffracted light of the operational order of diffraction.
However, diffracted light of the adjunctive orders of diffraction of the positive side is more influential than that of the negative side in terms of flare and ghost so that the optical performance of known scanning optical devices can be degraded because of the following reasons.
(i) The extent of exposure is raised relative to the operational order of diffraction because of a slow scanning rate.
(ii) Diffraction is directed inwardly (and close to the optical axis of the scanning optical system) relative to the operational order of diffraction so that diffracted light inevitably enters the effective image area of the surface to be scanned. On the other hand, diffracted light of the adjunctive orders of diffraction of the negative side leaves the effective image area at a position near the position where the light beam remotest from the optical axis passes and diffracted light of the adjunctive orders is found to a large extent.
In view of the above described circumstances, it is therefore an object of the present invention to provide a scanning optical device that is free from the above identified problems and has a configuration that is simple but can enhance the uniformity of field illumination on the surface to be scanned and minimize the fluctuations of aberration due to various changes by reducing the influence of flare and ghost,
Another object of the present invention is to provide a high definition image forming apparatus comprising a scanning optical device according to the invention and adapted to produce high quality images.
According to the invention, the above objects are achieved by providing a scanning optical device comprising:
a light source;
an optical deflector for deflecting the light beam emitted from said light source;
a first optical system for leading the light beam emitted from said light source to said optical deflector; and
a second optical system for focussing the light beam deflected by said optical deflector on a surface to be scanned;
said second optical system having at least a diffraction optical element and being adapted to form a light spot on the surface to be scanned by using the diffracted light beam of a predetermined order of diffraction out of the light beams diffracted by the diffraction optical element, said diffraction optical element being so configured as to make the sum of the quantities of light of the diffracted light beams of the orders of diffraction of the positive side relative to the predetermined order of diffraction smaller than the sum of the quantities of light of the diffracted light beams of the orders of diffraction of the negative side relative to the predetermined order of diffraction for the light beams deflected by the optical deflector and located remotest from the optical axis.
In another aspect of the invention, there is also provided a scanning optical device comprising:
a light source;
an optical deflector for deflecting the light beam emitted from said light source;
a first optical system for leading the light beam emitted from said light source to said optical deflector; and
a second optical system for focussing the light beam deflected by said optical deflector on a surface to be scanned;
said second optical system having at least a diffraction optical element and being adapted to form a light spot on the surface to be scanned by using the diffracted light beam of a predetermined order of diffraction out of the light beams diffracted by the diffraction optical element, said diffraction optical element being so configured as to make the sum of the quantities of light of the diffracted light beams of the orders of diffraction of the positive side relative to the predetermined order of diffraction smaller than the sum of the quantities of light of the diffracted light beams of the orders of diffraction of the negative side relative to the predetermined order of diffraction for any light beams located within the scope of scanning.
In still another aspect of the invention, there is also provided a scanning optical device comprising:
a light source;
an optical deflector for deflecting the light beam emitted from said light source;
a first optical system for leading the light beam emitted from said light source to said optical deflector; and
a second optical system for focussing the light beam deflected by said optical deflector on a surface to be scanned;
said second optical system having at least a diffraction optical element and being adapted to form a light spot on the surface to be scanned by using the diffracted light beam of a predetermined order of diffraction out of the light beams diffracted by the diffraction optical element, said diffraction optical element being so configured as to satisfy the requirement of the formula below in terms of the light beams deflected by the optical deflector and located remotest from the optical axis;
0.5 less than (Pmxe2x88x921/Vmxe2x88x921)/(Pm+1/Vm+1) less than 2.0,
m being the predetermined order of diffraction, Px being the intensity of a diffracted light beam of the x-th order of diffraction, Vx being the scanning speed of a diffracted light beam of the x-th order of diffraction on the surface to be scanned.
In still another aspect of the invention, there is provided an image forming apparatus comprising:
a scanning optical device having the above features;
a photosensitive member arranged at the surface to be scanned;
a developing unit for developing an electrostatic latent image formed on the surface of said photosensitive member by the light beams made to scan the surface by means of said scanning optical device into a toner image;
a transfer unit for transferring said developed toner image onto a toner image receiving member; and
a fixing unit for fixing the transferred toner image on the toner image receiving member.